A Guide To Raft Foundations

When constructing any type of structure, having the right foundations in place is integral. 

The purpose of foundations is to provide a solid base for the structure to be built upon, and a place where the structure’s weight can be evenly distributed to ensure it is kept structurally sound and, ultimately, is not going to fall down.

It is all about safety, and if you do not have the right kind of foundations in place then the safety and integrity of the entire structure will be called into question. 

There is a wide range of foundation types, and it is important to understand the benefits of each kind before starting out on a construction project. 

Raft foundations are a popular foundation type, and in this article, the team here at Concrete Supply UK will outline everything you need to know about them.  

Read on to find out more…

What Is A Raft Foundation? 

If you’re wondering what a raft foundation is, we’re here to help. 

In short, raft foundations is a type of foundation that offers a strong concrete base, and that is typically used to hold up various large-scale structures including buildings. Bridges, tanks and other major applications. They feature a reinforced concrete slab that is placed on the ground and where the main structure is going to be built on top of it. 

Often, this foundation is described as ‘floating’ on the ground in much the same way that a raft floats on water and it is for this reason that it gets its name. 

These foundations tend to be extremely thick and cover the entire area of a building’s footprint. This means that the weight of the structure is then spread as evenly as possible across the foundation, through the concrete and into the ground below it. Raft foundations actually spread the weight over a far greater area than other foundation types, and this also helps to lower pressure on the ground. It is often selected as a foundation type in an area where it is not suitable or appropriate to use traditional strip foundations or trench-fill foundations. 

Normally, raft foundations are used in environments that do not have strong soil – such as clay or sand. In these settings, it is important to reinforce the foundations in a way that will not see it sink into the ground and it is here where raft foundations are chosen as other foundation types will likely not work.  

So, if you were wondering ‘what is raft foundation?’ now you know. There is a wide range of raft foundation types, and we’ll examine these in the section below. 

Different Types Of Raft Foundations 

The various raft foundation types include the following:

Solid Slab Raft Foundations 

Solid slab raft foundations fall into various subcategories which all offer different benefits. 

These include:

• Flat raft mats: This is a reinforced concrete slab of consistent and uniform thickness with a mass of steel reinforcement installed on the top and the bottom. This helps to provide resistance against both upward and downward bending. 
• Wide toe raft: This type of raft foundation is used when the ground needs a thick, flat raft of concrete. This might be in environments where the ground has particularly poor compressibility. The edges of the raft that support the main walls are built thicker and deeper and feature reinforced cages. Construction of these types is similar to thick concrete floor slabs but they can be cast straight into the ground or take on the ‘floating’ effect as mentioned earlier. Usually, a thicker beam will be used around the foundation’s perimeter to handle the heavier load of external walls.
• Slip plane rafts: For slip plane rafts, a layer of compressed sand beneath the foundation is used to handle any movement of the surrounding ground.
• Blanket rafts: Similar to slip plane rafts, slip plane rafts use layers of stone to help the foundation handle weak spots in the ground. 

Slab beam raft foundations 

When the structure is going to be unusually large or uneven, then slab beam raft foundations may be required. They provide additional support to the structure, using beams that are set up directly beneath the structure. 

Cellular raft foundations 

If heavy loads are imposed on ground that has an unusually low load-bearing capacity, then cellular raft foundations may be required. Usually, this involves using two slabs with interlocking ground beams that run in different directions. This makes them very strong and rigid and suitable for keeping the structure upright.

Piled raft foundations 

A standard raft slab that has been enhanced, piled raft foundations bring extra stability and improve the load capacity of the foundations. These types tend to be used for taller buildings or when the site has a particularly low load bearing capacity.    

Benefits of raft foundations 

There are several benefits of raft foundations including the following:

Strengthening areas of bad soil: If you’re building a structure in an area with sub-standard soil, or with clay and sand where subsidence can occur, it’s important to reinforce the area before any construction takes place. Raft foundations are the best option in these instances, as the slab used will be able to handle such problems. Plus, other shallow foundation types tend not to work in these environments. 

Evenly spread the load: Some foundation types concentrate the load in a small area, but with raft foundations, the weight is shared across a large surface area (usually the entire blueprint of the structure), this allows for an evenly spread room that enhances the sturdiness of the base. 

Cost-effective: Raft foundations are also often cheaper than other foundation types, because they do not require as much digging into the ground as other options. 

Here at Concrete Supply UK, we’re able to supply our customers nationwide with quality types of concrete mixes including those that are suitable for foundations whether for domestic or commercial construction projects.

This includes raft foundations, and we’ll always formulate the concrete applications and grades based on your project’s unique requirements ensuring you have the right quantities as well as the right strength and grade to achieve your goals.

Contact our team today to find out more.